I found this article by Pamela Prindle Fierro (About.com) helpful to explain the science behind why Jeanne and I look and act differently.
(Here’s part of the full article)
“The stereotype of identical twins is that they are exactly alike: they look alike, they dress in matching outfits, they share the same likes and dislikes. Parents of identical twins know differently, however. Despite their shared genetic component, identical multiples are unique individuals. Though they do share similarities, they also have many differences.
For example, my own children have always exhibited about a twenty-five percent difference in their weight. When they were newborns, weighing four and five pounds, it was quite obvious. At other times as they’ve grown up, it’s not noticeable. We have confirmed that they are indeed identical twins, yet people are often skeptical because they don’t “look” alike.
They don’t act alike either. One likes to dance; the other likes to play basketball. Certainly, we encourage them to pursue their individual interests, but the initial inclination towards these activities were all their own.
What are Identical Twins?
Identical, or monozygotic, twins develop from a single egg/sperm combination that splits a few days after conception. Their DNA originates from a single source, thus their genetic makeup is the same and the characteristics that are determined by genetics will be similar. Monozygotic twins are always of the same gender, except in extremely rare cases of chromosomal defect.
On the other hand, fraternal, or dizygotic, multiples form when two separate eggs are fertilized by separate sperm in a single ovulation cycle. They are no more alike than any sibling set, sharing about 50% of their genetic markers in a unique combination of genes from both parents.
While identical twins form with the same set of genes, human development is not just genetic. The environment also has an impact. So, beginning in the early environment of the womb, external influences can change the appearance of twins. For example, some monozygotic twins share a placenta. One twin may have a more advantageous connection to the placenta, receiving the first run of nutrients. This situation can cause a size discrepancy between the babies, a physical difference that continues as they grow up. Twin-to-Twin Transfusion Syndrome (TTTS) is another condition that affects twins in the womb, and can impact their development.
While most twins grow up in the same home environment, there are many circumstances that create differences in the childrens’ appearances, personalities, and interests. As the twins approach the teen years, they may even seek to establish dissimilar qualities in order to establish individual identities.”